Hi James I'm a professional electronics designer, 2 months from "retirement" age after being in the game for at least 40 years am and still learning every day! When I stop learning I think it'll be time to stop working. RP On 25 August 2017 at 19:59, James Burkart wrote: > I've got so much to learn. Sean, what's your education and background? I'= m > a junior working on my EET and only have a hobbyist background. > > -- > Sincerely, > > James Burkart > *Filmmaker & Documentarian* > > *Burkart Studios* > 925.667.7175 | Personal > 415.738.2071 | Office > > *Web:* burkartstudios.com > *Facebook:* facebook.com/burkartstudios > > On Thu, Aug 24, 2017 at 9:28 PM, Sean Breheny wrote: > > > Hi Bob, > > > > My main suggestion is to characterize the behavior by providing various > > input signals like impulses, steps, white noise, swept sine, etc. and > > looking at the output on a scope. You might also vary the output load. > This > > gives you a good idea that you are not close to going unstable and you > > aren't experiencing "peaking" around one frequency band or a null in on= e > > band. > > > > I've had problems in the past with circuits like this where the output > > stage has a nonlinearity about zero (like crossover distortion) which c= an > > make the gain be reduced substantially near zero. This can result in > > instability by causing integrator wind-up while the output is in the > > low-gain region, followed by massive overshoot when it exits the low-ga= in > > region, followed by a compensatory wind-up in the other direction due t= o > > the overshoot, etc. This problem can be dealt with by making sure that > the > > transistors in the output are always biased with some minimum current s= o > > their input-output gain never goes below a certain value. > > > > Note, too, that the op-amp can be treated as it's own finite gain stage= , > > where you close the loop locally around it and then close the loop agai= n > > around the whole system. This can make analysis of the entire loop easi= er > > because once you guarantee that the op-amp gain stage is itself stable, > you > > can then treat it as a pure gain block or as a simple compensator (like > an > > integrator or integrator plus proportional gain or a lead compensator o= r > > lead-lag compensator, etc.) You are sorta doing that already in that th= e > > compensation cap is turning the op-amp into a fast integrator. > > > > I have developed a "theorem" of simple control design (which I am sure > has > > been stated before but I've never seen it put this way) - given any > stable > > system P, one can always close the loop around P with an integrator H > such > > that the closed-loop system will have zero DC error and be stable, for > some > > value of integrator gain K in H, and the system will continue to be > stable > > and exhibit zero DC error for any integrator gain Kprime < K. To put it > > more simply, you can always control a stable plant using a slow > integrator > > and obtain at least the improvement of zero DC error, but also usually > some > > amount of improvement in servo tracking and regulation against > > disturbances. > > > > Sean > > > > > > On Thu, Aug 24, 2017 at 12:32 PM, Bob Blick > wrote: > > > > > Hi y'all, > > > > > > I'm trying to build some audio power amplifiers in order to use up so= me > > > parts I have left over from a previous life. > > > > > > In the past I've always built the input stages from discrete parts bu= t > > > this one uses a conventional 4558-style opamp. Then I follow it with > > > voltage and current boosting parts and finally enclose the whole mess > in > > > negative feedback. > > > > > > In this aforementioned past life I designed a lot of power amps so I > know > > > to design each stage to have as linear and balanced response as > > reasonable, > > > keep the gain low around the slowest components etc. > > > > > > Of course with the added gain and relatively low speed of the output > > > stage, the internally compensated opamp is not able to keep things > > stable, > > > so I've added an external compensation capacitor. It does work fine, > and > > > I'm not looking for super high fidelity, but it's always nice to make > > > simple changes that reap big benefits. > > > > > > So my question is, any ideas about compensation or changes I could > make? > > > > > > I've attached a block diagram. Note that my output stage is the > > right-hand > > > amplifier block, it contains many discrete components, and most > > > importantly, it is inverting. Therefore I am using the non-inverting > > input > > > of the opamp for the negative feedback. > > > > > > Thank you! > > > > > > Bob > > > -- > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > > View/change your membership options at > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .